1. Field of the Invention
The invention relates to a device for optically scanning a record carrier information plane having tracks, which device comprises an optical system for generating a first and a second tracking beam and a main beam, a lens system for focusing the three beams on the information plane to two tracking spots at both sides of the centreline of a track to be scanned and to one main spot on said track, at least three detection systems a, b and c for receiving radiation of the first and second tracking beams and the main beam from the information plane, and a signal processing circuit for deriving a reference signal S.sub.r from detector signals S.sub.i, where detector signal S.sub.i is a measure of the total radiation energy incident on detection system i,
2. Description of the Related Art
A device of this type which can be used, inter alia, in apparatuses for writing and reading information in the tracks of optical record carriers is known from U.S. Pat. No. 4,446,545. In this device two tracking beams are focused to tracking spots at both sides of the track on which a main beam is focused to a main spot, and it comprises three detection systems in the form of single detectors a, b and c for detecting the intensity of each one of the tracking beams and the main beam after reflection on the information plane. The difference of the output signals S.sub.a and S.sub.b of the detectors a and b, respectively, is the tracking error signal which can be used as a control signal for the tracking servosystem with which the main beam is held on the track. To be independent of intensity variations of the radiation source, the tracking error signal is normalized by dividing it by a reference signal S.sub.r. The known reference signal is generated in accordance with the formula EQU S.sub.r =S.sub.a +S.sub.b -.alpha.S.sub.c,
in which the constant .alpha. is larger than 0 and is chosen to be such that the reference signal has a cosine-shaped variation as a function of the distance between the main spot and the centreline of the track. Since the non-normalized tracking error signal has a sine-shaped variation, the normalized tracking error signal will thus have a tangent-shaped variation.
A drawback of the known reference signal is that the signal S.sub.r becomes periodically zero when the beams move transversally across the tracks, for example, during a searching action, so that problems with the desired division will arise if no further measures are taken. Moreover, the cosine-shaped variation of the reference signal can only be used for normalizing the tracking error signal. Other control signals such as a focus error signal cannot be normalized by means of the known reference signal.